Solid rocket propellant must be designed, evaluated and produced so as to evolve or generate hot gas in a controllable manner. This controlled evolvement of hot gas can then be utilized to propel a missile, rocket or other projectile in a predictable way.
It is well known to those skilled in the technology that to ensure controlled gas evolution (burning), the following ballistic performance parameters of the propellant must be measured:
(1) burn rate (r) as a function of pressure, generally given by (r) in the equation r=aP.sup.n, where P is the pressure;
(2) burn rate exponent, generally given by (n) in the same equation;
(3) burn rate pre-factor, generally given by (a) in the same equation;
(4) burn rate sensitivity to temperature given by ##EQU1## written as .sigma..sub.p ; and
(5) pressure sensitivity with respect to area ratio where the area ratio is defined as K=propellant burning surface area divided by rocket nozzle throat area and the pressure sensitivity is defined as: ##EQU2## There are two methods used to measure these quantities. The first is called strand burning. Well-known to those skilled in the art is the Chemical Propulsion Information Agency (CPIA) handbook which contains standard data for strand burning of various propellants, some of which data is contained in graph form. This method consists of cutting the propellant into spaghetti-size strands and then burning them at various constant temperatures and pressures. The strands must be burned in an expensive device, a Crawford Bomb, which requires much maintenance. Additionally, many strands must be burned (requiring multiple test burns) to collect the data required to evaluate parameters (1) through (4) above. This procedure is very time consuming and expensive. Parameter (5) above cannot be evaluated by the strand burning method.
Further, this strand burning method does not allow testing of the propellant under the actual conditions inside a rocket motor. Although the strands are brought to the required pressure by external means, such as nitrogen pressure, and then ignited and burned, this environment does not simulate the turbulent conditions the propellant actually sees inside a rocket motor.
The second method uses a Ballistic Evaluation Motor (BEM). This motor has two advantages over strand burning. First, the BEM allows evaluation of all five parameters, not just (1) through (4). Secondly, the propellant can be evaluated in an environment that simulates conditions inside a rocket motor. This simulation is not possible with strand burning.
Although, there are several types of BEM's used by those familiar with propellant evolution technology, none of the currently available types allow testing at pressure ranges between 5,000 and 10,000 psi. Additionally, new propellants containing aluminum and other corrosive agents, especially when combined with more energetic fuels, have rendered current BEMs unsuitable.
In fact, slab motor designs that provided burning rate characteristics in years past, i.e. the Production Slab Motor and the Advanced Slab Motor, are not adaptable to the newer propellants now under development, especially those containing aluminum and other corrosive agents. The Production Motor was constructed of 1010/1020 steel and used zinc chromate putty to seal mating surfaces. The Advanced Motor is identical except for an asbestos insert and forward and aft insulators. The zinc chromate putty cannot withstand the higher temperatures and pressures encountered with new propellants. As a result, gas flow escaping around the motor tube and around the nozzle shell typically erodes the steel hardware and, in some tests, burns completely through the asbestos inserts. In such a situation, the test data is invalid, and, in addition, the damage causes a drastic depletion of the hardware inventory.
There is an ongoing need to determine burning rate characteristics of energetic and corrosive propellant mixes during research and development. Further, testing is required to determine the effects of these propellants on the numerous rocket motors already in the fleet. The test vehicle must also allow determination of specification compliance of numerous grains to establish and control lot acceptance criteria.